Process for electrodepositing manganese metal

ABSTRACT

A PROCESS FOR ELECTRODEPOSITING MANGANESE METAL WHEREIN A SELENIUM COMPOUND IS ADDED TO A CONVENTIONAL MANGANESE METAL ELECTROLYTE TO INCREASE THE CURRENT EFFICIENCY OF THE CELL AND A SMALL QUANTITY OF SULFUR DIOXIDE IS INTRODUCED TO MAINTAIN THE SELENIUM COMPOUND IN SOLUTION IN THE ELECTROLYTE.

United States Patent 3,696,011 PROCESS FOR ELECTRODEPOSITING MANGANESE METAL San-Cheng Lai, Oklahoma City, Okla., assignor to Kerr-McGee Chemical Corp., Oklahoma City, Okla. N0 Drawing. Filed Oct. 28, 1970, Ser. No. 84,936 Int. Cl. C22d N24 US. Cl. 204-105 M 3 Claims ABSTRACT OF THE DISCLOSURE A process for electrodepositing manganese metal wherein a selenium compound is added to a conventional manganese metal electrolyte to increase the current efiiciency of the cell and a small quantity of sulfur dioxide is introduced to maintain the selenium compound in solution in the electrolyte.

BACKGROUND OF THE INVENTION It is well known that in the electrodeposition of manganese metal, the addition of a small amount of sulfur dioxide to the electrolyte helps to prevent oxidation of the manganous ion in the solution and reduces the undesired formation of manganese dioxide on the anode. However, in commercial practice when sulfur dioxide is used as an additive, it has been found that current efficiencies in the cell are only about 60-65%.

In an article Effect of Selenious Acid on the Electrodeposition of Manganese, by A. I. Rao, Y. D. P. Rao, and R. Vedaraman, published in the November-December 1966 (volume 4) issue of the Journal of Electrochemical Technology, it was reported that when selenious acid was used as an additive in place of sulfur dioxide in the electrodeposition of manganese metal that current efficiencies in excess of 90% could be obtained. Further, it has been reported that higher current densities can be used when selenium is used as the additive and that its use results in lower cell voltages when compared with conventional sulfate bath electrolytes wherein sulfur dioxide is used as the addition agent.

Summary of the invention It now has been discovered that if a selenium compound is used as the sole additive, an orange precipitate or colloid or amorphous selenium metal forms in the electrolyte. This is disadvantageous, of course, because the selenium is thereby removed from the electrolyte solution and the advantages of its presence are lost. Further, costs are increased because of the necessity of having to replace the selenium which has precipitated out of solution.

Furthermore, the formation of the precipitate is disadvantageous because in the commercial electrodeposition of manganese metal, the electrolyte is introduced into the catholyte chamber of an electrolytic cell where the manganese is precipitated on the cathode. The catholyte then flows through a diaphragm into the anolyte compartment of the cell. The anolyte then is removed for purification before recycling to the catholyte compartment. When a selenium compound is used in such a process, it has been found that the selenium precipitate tends to deposit upon and plug the openings in the diaphragm of the cell, necessitating shutdown to either clean or replace the diaphragm.

The present invention is based upon the surprising discovery that a selenium precipitate does not form if a small amount of sulfur dioxide also is introduced into the electrolyte.

The selenium compound is added in amounts sufiicient to obtain optimum current efiiciencies and a deposit of 3,696,011 Patented Oct. 3, 1972 DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred electrolyte for the practice of this invention is ammonium sulfate containing a source of manganese. The best results have been obtained when the electrolyte was maintained at a pH of from about 7-8.5 and contained ammonium sulfate in an amount of from about to 160 gm./liter and from about 11-45 gm./liter of manganous ion.

The selenium compounds which are suitable for use in the present invention are selenious acid (H SeO or other selenium compounds such as for example 8e0 5e0 Na SeO (NH SeO Na SeO (NH SeO and the like. These compounds may be added to the electrolyte directly or fed in continuously as aqueous solutions. The concentration of selenium may be present in an amount of from 0.01-0.5 gm./liter. However, it is preferred that the concentration of selenium be from about 0.03 gm./ liter to 0.1 gm./liter.

The sulfur dioxide may be added to the electrolyte by bubbling liquified or gaseous S0 into the electrolyte. It also may be supplied by adding to the electrolyte compounds such as H SO Na SO (NH4)2SO3 and like which may be converted to S0 in the electrolyte. The concentration of S0 in the electrolyte should be sufficient to maintain the selenium compound in solution. Based on the quantity of selenium set forth herein, it has been found that the S0 concentration should be maintained within the range of from about 0.01 to 0.1 gm./ liter. However, the best results have thus far been obtained when the S0 is present in an amount of from about 0.01 to 0.05 gm./liter.

EXAMPLE To demonstrate the effectiveness of the present invention, a series of electrodepositions of manganese metal were carried out under the following conditions. The electrolyte had a:

pH--8.2 Temp.-35 C. Current density-40 a.s.f.

The initial electrolyte consisted of:

Gm./l. Mn++ 35 (NH SO The results of these tests are shown in the following table.

From these tests it will be seen that the addition of S0 to the electrolyte substantially reduces the precipitate formed in the catholyte. Further, it enhances the current efficicncies of the cell when used Within the preferred range of from about .01 to .05 gm./-liter. The presence of S0 in amounts of 0.1 gm./liter and greater in the electrolyte result in the formation of a precipitate. The precipitate is believed to result from the reaction of the S0 with the manganous salt solution to form manganese ,4 sulfite in an amount in excess of its solubility in the pound is selected from the group consisting of H SeO electrolyte. 8e0 (NHg seO (NH SeO and SeO EXAMPLE Se in the electrolyte, gm./liter SOzin the electrolyte, gm./liter. 0 0.01 0.05 0.1 0 0.01 0.05 0.1 0.00 0.01 0.05 0 0.001 0.05 0.1 Precipitate formed in the 0atholyte,gm./liter 0.055 0.0008 0. 0008 0.050 0.08 0.001 0.001 0.06 0.08 0. 001 0. 001 0.095 0.0075 0.0082 0. 074

Current etfieiency, percent 89 90.5 90 89.4 92 93.8 94.5 90 92.0 93.8 94.5 90 92 92.4 89.5

1 Orange precipitate appears in the catholyte.

2 Clear catholyte. i

What is claimed is: 3. The process of claim 1 wherein said selenium is 1. In a process for electrodepositing manganese metal pr s nt in an amount of about 0.1 gm. per liter of elec from an ammonium sulfate electrolyte containing man- 15 tfolyte and the Sulfur dlOXlde 1S P 111 an u t o ganese, the improvements which comprise introducing from about 001 to 8 P lltel' of electrolyteinto said electrolyte a selenium compound in an amount R f d sufficient to provide from about 0.01 to 0.5 gm./liter of e erences selenium, based on the total volume of the electrolyte Effect Of Selenlolls Acld 011 the Electfodeposltloll 05 and effecting the deposition in the presence of sulfur 20 M11, y ell ElectrochemlcalTechnology, Novemdioxide in an amount within the range of from about her-December PP- 0.01 to 0.10 gm./liter to maintain the selenuim compound k in Solution in said e1 6 ctrolyw HOWARD S. WILLIAMS, Primary Examiner 2. The process of claim 1 wherein the selenium com- R ANDREWS, Assistant Examiner 

